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mand reports that FP 4130 potentially
can reduce product cost and weight and
enhance mechanical performance.
Metallic armors typically work best
against bullets and blast fragments.
However, ballistic tests at the U.S. Army
Aberdeen Test Center show that 6.3-mm
thick FP 4130 with a 50 kg/m2 area density was the highest performing metallic
armor against bullets and blast fragments at that mass. The material
stopped higher velocity armor piercing
bullets and blast fragments better than
more costly aluminum, magnesium, and
titanium armors now in use.
700
Transformation 1
(bainite)
650
HTPRO
x: -3.222
y: 648.9
Temperature, °C
x: 0.01079
y: 549.8
14
550
500
450
Transformation 2
(martensite)
x: -0.1943
y: 459.4
400
350
x: 20.04
y: 358.8
300
0
5
10
T, °C
15
20
Fig. 5 — SSDTA curve shows two transformation events representing the formation of
martensite and bainite.
Martensite
Frequency
1000
500
Bainite
0
400
450
500
550
600
Hardness, HV
650
700
750
Fig. 6 — Microindentation hardness histogram of flash processed AISI 4130 shows two
hardness peaks indicative of approximately 82.5% martensite and 17.5% bainite.
processing could have significant impact
in various industrial applications. Those
requiring high strength and toughness,
such as armor, and automotive components requiring good crash resistance
would benefit most.
nite present throughout an entire FP
sample via microscopy due to extremely
small feature size and intermixing of
phases. An SSDTA curve produced
during rapid quenching in Region IV
for FP AISI 4130[1] (Fig.5) shows two
transformation events: the formation of
martensite and bainite in FP due to the
transformation temperatures and high
cooling rate. A histogram of microindentation hardness values measured
over the entire sample cross section
(Fig. 6) shows two hardness peaks corresponding to the presence of approximately 82.5% of a harder phase and
17.5% of a softer phase. In the case of
FP AISI 4130, phase fractions were determined to represent 82.5% martensite
and 17.5% bainite. Research shows that
martensite and bainite fractions can be
adjusted with different processing conditions, initial microstructure, and steel
composition. FP produces negligible
values of retained austenite.
Industrial impact
The properties produced using flash
50
In the automotive industry, many OEMs
and Tier 1 automotive suppliers find that
FP steels reduce component weight
without compromising performance.
Hyundai Motor Group recently tested
FP steels for use in applications such as
vehicle door-side impact beams. In a
drop weight test with a mass of 320 kg
and an impact velocity of 5 m/s[2], FP
tubing outperformed the industry standard boron-steel tubing by 20% in total
bending energy absorbed, and by 15% in
resisting bending force. Mathematical
modeling shows FP steel in automotive
bumpers and trailer hitches meet OEM
performance requirements at 67% of the
weight of current materials.
In armor applications, the U.S. Army Research Development Engineering Com-
ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013
FP steel has good weldability because it
does not require pre- or post-weld heat
treating required for other armors with
similar ballistic resistance. Softening occurs in the HAZ (also common in Al, Ti,
and Mg armors), which is detrimental to
ballistic resistance, but vehicles are often
built with overlapping joints, which
could negate softening effects. In addition, due to its reduced thickness, FP
steel can be welded in a single pass compared with other lower density materials, which may necessitate multipass
welding due to increased thicknesses required to provide the same strength. FP
steels may be easier to fabricate and handle due to greater familiarity with the
steel grades versus other materials.
Current developments
Research in flash processing is currently underway to fully exploit the
potential of FP steels and their applications. Researchers at The Ohio State
University are examining FP steel weldability with respect to strength and ballistic requirements, exploring strategies
to mitigate softening at elevated temperatures, and creating a model with
the ability to predict material properties
for different applications. HTPRO
References
1. T. Lolla, B. Alexandrov, S.S. Babu, and G.
Cola, Towards Understanding the Microstructure Development During Flash
Heating and Cooling of Steels, Materials
Science and Technology, 2008.
2. G.M. Cola, Jr., Flash Bainite Process, Adventures in the Physical Metallurgy of
Steels, 2013.
For more information: S.S. Babu, Department of Mechanical, Aerospace & Biomedical Engineering, The University of
Tennessee, Knoxville, sbabu@utk.edu,
www.engr.utk.edu.